Project Summary: Water and solute exchange between streams and the surrounding subsurface (hyporheic exchange) has recently been recognized as a critical process in the cycling of many important substances in watersheds. Hyporheic exchange has particularly been shown to influence nutrient and carbon dynamics, and the fate and transport of contaminants in surface waters. These issues have risen to the forefront in the Midwestern U.S. because substantial nitrate loads from agriculture and urban centers contribute to chronic hypoxia in the Gulf of Mexico. Despite the clear link that exists between hydrologic processes in headwater streams, denitrification efficiency in removing stream nitrate, and nitrogen loads reaching the Gulf, our current understanding of these processes in low-gradient, sand-bed streams is largely empirical. Most glaring is the lack of, fundamentally-based, predictive models for hyporheic exchange in sand-bed streams, which has prevented the transfer of experimental observations gained at one site to other sites, or even to the same site under different flow conditions. Our objective is to collect a definitive data set from a low-gradient, sand-bed stream and test the applicability of several fundamental, process based models of hyporheic exchange that have proven successful in laboratory flumes. We seek to both advance our fundamental understanding of hyporheic exchange processes in real stream, but also to assess the improvements in prediction of solute transport that are possible for a variety of flow and geomorphic conditions. The stream selected for intensive study is Sugar Creek, a headwater stream in the agricultural region along the northern Illinois-Indiana border. Sugar Creek is representative of many low-gradient streams in agricultural headwater areas that are tributary to the Illinois and Mississippi River systems. We propose to conduct detailed tracer experiments and related physical measurements that will be notable because they will resolve hyporheic flow paths, solute transport, and controlling processes with an unprecedented level of detail. In-stream solute injections will be undertaken in several seasons in a 3 to 4 kilometer long reach of Sugar Creek. Stream-tracer data will be used to assess bulk transport integrated over 8 to 10 sub-reaches (50 300 m long) representing variable geomorphic conditions. Several of those sub-reaches will be thoroughly characterized in terms of their streambed topography and morphology, cross-channel flow variability, sediment characteristics, and subsurface movement of the solute tracer. This unprecedented level of detail will be made possible using state of the art measurement technologies. The resulting data sets will support the application of a suite of fundamental, predictive, process-based models of hyporheic exchange and its effects on downstream solute transport. The models will be evaluated in terms of their ability to predict not only reach-averaged tracer concentration data, but also point estimates of interfacial hyporheic flux and porewater tracer concentrations. Based on fundamental theory, we will develop and test approaches for upscaling rates of hyporheic exchange to predict solute transport in the entire study reach under seasonally varying conditions. In sum, we will apply several modeling approaches of differing sophistication in order to 1) evaluate our current ability to predict hyporheic exchange in sand bed streams from first principles, and 2) develop reasonable approaches for upscaling the computations so that solute transport can be predicted with a specified level of uncertainty for longer stream reaches. Because this study is focused on fundamental processes that occur widely, this work will have very broad scientific impacts and will be used to address multiple pressing societal concerns. Detailed understanding of the relationship between stream/sedimentary conditions and hyporheic exchange will facilitate improved understanding of nutrient dynamics, carbon cycling, and releases from contaminated sediments. Our goals for predictions (in terms of uncertainty and spatial resolution) are compatible with future applications in reach and basin-scale water-quality models. In particular, the choice of Sugar Creek as the intensive study site will provide critical hydrologic information to support complementary studies on nitrate fluxes and sedimentary denitrification in the headwaters of the Mississippi River basin. The project's broader impacts will also be increased by the synergistic activities of the PI's. Both PI's have a strong history of involvement in scientific organizations, and the project's themes will be reflected in activities such as special sessions at major technical meetings. We will particularly seek to broaden the general contribution of this work by encouraging interdisciplinary and international communication and inter-site comparisons. The project will also contribute considerably to human resource development, both through direct training of students and via the use of the field site for educational demonstrations.
